Recently, electronic technology has been remarkably developed, and various appliances have been made smaller and lighter. Along with the miniaturization and reduction in weight of electronic appliances, miniaturization and reduction in weight of batteries, serving as power sources of these electronic appliances, have been demanded. As batteries that have small volume and mass but are capable of providing large amounts of energy, non-aqueous electrolyte secondary batteries using lithium have been used. In addition, it has been proposed to use non-aqueous electrolyte secondary batteries as power sources for hybrid cars, electric cars, and the like, and the non-aqueous electrolyte secondary batteries have been put into practical use.
Generally, a non-aqueous electrolyte secondary battery has a positive electrode, a negative electrode, and a separator provided therebetween for insulating the positive electrode and the negative electrode. Conventionally, a porous film of a polyolefin-based polymer has been used as a separator used in the non-aqueous electrolyte secondary battery.
In the non-aqueous electrolyte secondary batteries, due to ions (in the case of a lithium-ion secondary battery, lithium ion (Li+)) moving between a positive electrode and a negative electrode through a separator, charging and discharging are possible. Therefore, the separator is required to not inhibit ions from moving freely, and a porous film having a plurality of microscopic holes has been used as the separator.
In addition, the separator is required to have a so-called shutdown function. The shutdown function is a function that improves safety of the non-aqueous electrolyte secondary battery by, in the case where a fine short circuit has occurred in a battery, inhibiting the movement of ions by blocking the holes in the part where the short circuit occurred in order to make the battery lose the function at the part. In the porous film of a polyolefin-based polymer, the shutdown function is achieved by, in the case where a fine short circuit occurred in the battery, melting the part where the short circuit occurred by increasing the temperature and thereby blocking the holes.
However, for example, if the battery temperature exceeds 150° C. due to an instant increase in temperature, the separator may instantly shrink and the part of the short circuit of the positive electrode and the negative electrode may expand. In this case, the battery temperature may reach several hundred degrees Celsius or higher, and it has been a safety problem of the non-aqueous electrolyte secondary battery.
To enhance the safety of the non-aqueous electrolyte secondary battery at high temperatures, it has been proposed to provide a porous film containing an inorganic material in between a conventional separator, such as a porous film of a polyolefin-based polymer, and a positive or negative electrode.
For example, it has been proposed to use a specific α-alumina when an inorganic oxide porous film having insulating properties is formed (e.g. see Patent Document 1). Patent Document 1 proposes to form an inorganic oxide porous film by applying an inorganic oxide slurry formed from α-alumina, a binder, and a solvent on an electrode or a separator, and drying the slurry. Furthermore, Patent Document 1 provides examples of various resins, as a binder, including polyvinylidene fluoride (PVDF).
The primary purpose of the invention described in Patent Document 1 is to provide an inorganic oxide porous material for use in a lithium-ion secondary battery that is thermally stable and highly uniform and has a suitable porosity from the perspective of electrical conductivity of lithium ion.
However, the investigation of the adhesion between an inorganic material and a binder resin that constitute a porous film containing the inorganic material was not sufficient.